1. Field of the Invention
The present disclosure relates to magnetic resonance systems generally, and more particularly, to an apparatus and methods for varying field strength in a magnetic resonance system while keeping a relatively uniform magnetic field distribution.
2. Discussion of Related Art
A need exists, particularly in developing countries, to reliably, inexpensively, and non-invasively measure iron stores within humans to help in the clinical management and treatment of a number of diseases, including hematological malignancies, liver disease, hemoglobinopathies, hereditary hemochromatosis, thalassemia, and sickle cell disease, that can be complicated and exacerbated by iron overload. For example, in combination with hereditary hemochromatosis and thalassemia, iron overload can cause heart failure, liver cancer, liver cirrhosis, arthropathy, and endocrine problems. In sickle-cell disease, iron overload contributes to increased mortality, organ failure, and liver cirrhosis, among other health effects. Detecting iron overload is important not only because of its prevalence and health effects, but because there exist effective therapies, such as chelators and other treatment regimens such as phlebotomy, for these health effects.
The traditional gold standard for assessing body iron stores in the liver is the liver iron concentration (“LIC”), as determined by biochemical analysis of a biopsy specimen. However, liver biopsy is highly invasive and painful, and carries a small but definite risk of severe bleeding. Because of biopsy's invasiveness, many clinicians rely on indirect iron measurements such as serum ferritin. However, serum ferritin is unreliable because it is affected by other factors such as inflammation, liver disease, infection, hemolysis, and other health effects.
Magnetic Resonance Imaging (“MRI”) is a technique for indirectly measuring liver iron levels non-invasively, through its effect on spin relaxation times of nearby water molecules. However, MRI liver iron measurements performed at a single field strength remain uncertain due to the complexity of the spin-relaxation mechanism, and its dependence on tissue properties other than the iron concentration itself. MRI measurements at two (or more) field strengths can help resolve this uncertainty since the spin-spin relaxation rate is weakly dependent on the applied static field, while the relaxation due to the presence of iron is strongly dependent on the static magnetic field. By varying the static field, it is possible to determine how much of the relaxation is due to the presence of iron and how much is due to spin-spin coupling. Currently, in order to make the two-field MRI measurements, one has to perform a measurement at one static field, remove the sample from the MRI unit, and place the sample into another MRI unit of different field strength. Since MRI units are expensive, many smaller hospitals and MRI centers may not have multiple MRI units and will not be able to perform the multiple-field measurements. Moreover whole-body MRI has disadvantages in cost and accessibility. An abdominal MRI scan cost at least $600 (in 2006 dollars), and scheduling the scan and waiting for the results of a single MRI can take days or weeks, with the costs and delays likely increased for two MRI scans. Additionally, current MRI machines are not easily portable.
It is however possible to simplify the magnetic resonance measurement if instead of acquiring images with a MRI machine, one aims to determine the liver iron concentration using just relaxation time measurements (i.e. MR relaxometry). Using an electromagnet whose field strength can be varied by controlling the current in the electromagnet, measurements of the spin relaxation time can be taken at two (or more) magnetic field strengths allowing for an accurate determination of liver iron concentration. However, electromagnets require a power supply and tend to be bulky compared to permanent magnets.
The Los Alamos National Laboratory developed a variable-field permanent magnet dipole (“VFPMD”), which is a c-shaped sector magnet with iron poles separated by a large block of magnet material (SmCo). Moving an iron shunt closer or further away from the back of the magnet, i.e., within a shunt gap formed in a base of the large block of magnet material, can continuously vary the central magnetic field from 0.07 T to 0.3 T. The iron shunt is specially shaped to make the dependence of the dipole field strength on the position of the iron shunt as linear as possible. The dipole has a 2.8 cm high by 8 cm wide aperture with poles that are about 10 cm long. There are several disadvantages associated with the VFPMD, however. First, it is unsuitable for use in clinical diagnostic systems. Rather, it is uniquely designed to meet the particular requirements of the Los Alamos Advanced Free-Electron Laser (“AFEL”) experiment, which requires many beam optics and diagnostic elements to be located in a limited area. Second, the configuration of the VFPMD and its method of operation produce significant external field strength, which is undesirable in clinical diagnostic applications because, due to the small size and application of the VFPMD, stray magnetic fields were not a concern. The external field strength produced by the VFPMD, however, is undesirable in clinical diagnostic applications.
A magnetic field adjusting apparatus and a magnetic field adjusting method are disclosed in U.S. Pat. No. 6,448,772 (the “772 patent”). The magnetic field adjusting apparatus disclosed therein is rather complex and uses linear programming to determine where a worker should dispose, as a second and final adjustment to magnetic field uniformity, a number of magnetic field adjusting pieces on a magnetic field generator. The number and location and number of the adjusting pieces that need to be moved are displayed on a display device. Disadvantageously, the method disclosed by the '772 patent relies on manually adding or subtracting small pieces of permanent magnet material. Consequently, adjustments to the magnetic field strength occur in discrete increments, or steps.
What are needed are a low-cost, compact, portable apparatus and methods for performing magnetic resonance imaging with accurate, variable, control of the magnetic field strength during a magnetic resonance measurement, while keeping a relatively uniform magnetic field distribution.